(CNN) --Assassinated Afghan opposition leader Ahmed Shah Massoud had "limited knowledge" of a planned attack against the United States and was warning the West of the threat, according to a newly declassified cable from the Pentagon's Defense Intelligence Agency.

Massoud, leader of the Northern Alliance, the Taliban's main opposition, was killed September 9, 2001, by a bomb inside a video camera during an interview at his headquarters with two Tunisian al Qaeda operatives posing as journalists.

The cable, written in November 2001, was obtained under the Freedom of Information Act by the National Security Archive at George Washington University in Washington, D.C. (Text of cable, PDF)

It was based on an interview with a classified source and reads:

"Through Northern Alliance intelligence efforts, the late commander Massoud gained limited knowledge regarding the intentions of the Saudi millionaire, Usama bin Laden and his terrorist organization, al-Qaida, to perform a terrorist act against the U.S., on a scale larger than the 1998 bombing of the U.S. embassies in Kenya and Tanzania."

The heavily edited DIA document does not specify what it meant by "limited knowledge," and the portion that follows the reference is blacked out.

It continues by referring to a speech Massoud gave to the European Parliament in April 2001 in which the cable says he "warned the US government" about bin Laden. Massoud was on a diplomatic trip to Europe seeking financial support for his cause from the EU and individual countries.

The DIA report points out that Massoud was not a military threat to al Qaeda, even though his forces were fighting the Taliban for control of Afghanistan.

"Our investigators did look into the matter during their recent travels [to Afghanistan] and spoke to persons who might have some knowledge about the subject," said a spokesman for the independent commission set up by Congress to investigate the September 11, 2001, terrorist attacks.

The cable says the two fake journalists, who were killed in the bomb blast, were al Qaeda operatives.

According to an article in Voice of Jihad, an online magazine the Middle East Media Research Institute says is associated with al Qaeda, the terrorist group claimed responsibility for Massoud's assassination.

The story appeared last week in a translated version of the magazine on the Web site of the Washington-based nonprofit independent institute, which provides translations of Arabic, Farsi and Hebrew media reports and analyses of trends in the region.

The article quoted an interview with a bin Laden bodyguard after word reached bin Laden's camp of Massoud's death:

"I remember asking him, 'What happened?' And he replied by saying that Sheikh Osama [bin Laden] asked the brothers: 'Who will take it upon himself to deal with Ahmad [Shah] Massoud for me, because he harmed Allah and his sons?' A few brothers volunteered to assassinate Massoud and be rewarded by Allah, and you heard the good news."

Several Tunisian men were convicted in Belgium in September of supplying false documents that Massoud's assassins used to help them travel to Afghanistan.